Hydrostatic transmission control system

ABSTRACT

An improved control system for a hydrostatic transmission includes an anti-stall system for preventing overloading of an engine which drives the hydrostatic transmission. The anti-stall system senses the speed at which the engine is operating, the input to output speed ratio of the transmission as indicated by the displacement of one unit of the hydrostatic transmission, and the setting of a throttle for the engine. The anti-stall system utilizes these three inputs to regulate the pressure of control fluid conducted to a speed control valve. The speed control valve is selectively operable to vary the pressure of control fluid conducted to a pressure responsive control motor which adjusts the displacement of a unit of the hydrostatic transmission. The anti-stall system varies the pressure of the control fluid conducted to the speed control valve with variations in the fluid pressure conducted to the control motor from the speed control valve. However in the event of an impending stalling of the engine, the anti-stall system reduces fluid pressure conducted to the speed control valve. The speed control valve opens in response to this decrease in pressure to quickly decrease the fluid pressure to which the control motor is exposed. This effects operation of the control motor to decrease the output speed of the hydrostatic transmission and thereby reduce the load on the engine.

United States Patent [191 Bojas Jan.8,1974

[ HYDROSTATIC TRANSMISSION CONTROL SYSTEM [75] Inventor: Edward J.Bojas, Marshall, Mich.

[73] Assignee: Eaton Corporation, Cleveland, Ohio [22] Filed: Mar. 22,1972 21 Appl. No.2 237,086

[52] U.S. Cl 60/19, 60/444, 60/445, 60/449, 60/465 [51] Int. Cl. Fl6h39/46 [58] Field of Search 60/19, 431, 443, 60/444, 445, 448, 449, 465

[56] References Cited UNITED STATES PATENTS 3,085,391 4/1963 Hatfield eta1. 60/449 X 3,139,723 7/1964 Hollowell 60/433 3,214,911 11/1965 Kempson60/431 3,371,479 3/1968 Yappet al. 60/431 3,528,243 9/1970 Cryder et al60/444 X 3,540,220 11/1970 Lauck 60/19 3,572,213 3/1971 Lauck 60/444 xPrimary Examiner-Edgar W. Geoghegan Attorney-Joseph R. Teagno et a1.

57 ABSTRACT An improved control system for a hydrostatic transmissionincludes an anti-stall system for preventing overloading of an enginewhich drives the hydrostatic transmission. The anti-stall system sensesthe speed at which the engine is operating, the input to output speedratio of the transmission as indicated by the displacement of one unitof the hydrostatic transmission, and the setting of a throttle for theengine. The antistall system utilizes these three inputs to regulate thepressure of control fluid conducted to a speed control valve. The speedcontrol valve is selectively operable to vary the pressure of controlfluid conducted to a pressure responsive control motor which adjusts thedisplacement of a unit of the hydrostatic transmission. The anti-stallsystem varies the pressure of the control fluid conducted to the speedcontrol valve with variations in the fluid pressure conducted to thecontrol motor from the speed control valve. However in the event of animpending stalling of the engine, the antistall system reduces fluidpressure conducted to the speed control valve. The speed control valveopens in response to this decrease in pressure to quickly decrease thefluid pressure to which the control motor is exposed. This effectsoperation of the control motor to decrease the output speed of thehydrostatic transmission and thereby reduce the load on the engine.

29 Claims, 4 Drawing Figures 1 HYDROSTATIC TRANSMISSION CONTROL SYSTEMBACKGROUND OF THE INVENTION The present invention relates generally toan apparatus for controlling the operation of a hydrostatic transmissionand more specifically to an apparatus for preventing overloading of aprime mover by the hydrostatic transmission. 7

Anti-stall systems have been utilized with hydrostatic transmissions toprevent an overloading of an associated prime mover or engine. Two knownanti-stall systems are disclosed in US. Pat. 'Nos. 3,533,230 and3,139,723. Both ofthese known anti-stall systems sense the output speedof an engine which drives the associated hydrostatic transmission andthe throttle setting for the engine. Upon the occurrence of animpendingoverloading or stalling of the engine, these known antistall systemseffect a reduction in the effective displacement of the hydrostatictransmission to reduce the transmission output speed and the load on theengine.

SUMMARY OF THE PRESENT INVENTION The present invention provides ananti-stall system for use-in association with a pressure regulator valvewhich supplies fluid to a pressure responsive control motor to vary theinput to outputspeed ratio of a hydrostatic transmission. The anti-stallsystem senses the operating speed of an engine which drives thetransmission, the setting of a throttle or accelerator for the engine,and the displacement of a variable displacement unit of the hydrostatictransmission. During normal operation of the engine and hydrostatictransmission, that is when an impending stall condition is not present,

the anti-stall system maintains the pressure of control fluid suppliedto the pressure regulator valve at a pressure which is just slightlygreater than the fluid pressure supplied from the valve to the controlmotor. When an impending stall condition is present, theanti-stallsystem' reduces the fluid pressure conducted to the pressureregulator valve. The pressure regulator valve opens in response to thisreduction in pressure to reduce thefluid pressure conducted to thecontrol motor. Since during normal operating condition, the anti-stallsystem maintains the pressure of a control fluid conducted to thepressure regulator valve at a pressure which is just slightly greaterthan the fluid pressure supplied to the control motor, the output speedof a hydrostatic transmission can be immediately reduced at any speedratio by merely effecting a relatively small reduction in the pressureof control fluid supplied to the pressure regulator valve.

Accordingly, it is an object of this invention to provide a new andimproved anti-stall system which is dependable and operates to quicklyreduce the output speed of a hydrostatic transmission under manydifferent operating conditions in response to an impending overloadingof an engine which drives the hydrostatic transmission. Another objectof this invention is to provide a new and improved apparatus forcontrolling the operation of a hydrostatic transmission which is drivenby an engine having a selectively settable throttle and wherein theapparatus includes an anti-stall system which senses the operating speedand throttle setting of the engine and the displacement of a variabledisplacement unit of the hydrostatic transmission.

Another object of this invention is to provide a new and improvedcontrol apparatus which includes a selectively operable pressureregulator valve for varying the pressure of fluid supplied to a controlmotor to adjust the input to output speed ratio of a hydrostatictransmission and an anti-stall system which reduces the pressure of thecontrol fluid supplied to the pressure regulator valve and to thecontrol motor to reduce the output speed of the hydrostatictransmission'in response to the occurrence of an impending overloadingof the engine and wherein the anti-stall system varies the pressure ofthe fluid supplied to the pressure regulator valve as a function ofvariations in the pressure of the control;

fluid supplied to the control motor in the absence of an impendingoverloading of the engine to enable the antistall system to respondquickly to the occurrence of an impending overloading 'of the engine.

Another object of this invention is to provide a new and improvedapparatus for controlling the operation of a hydrostatic transmissiondriven by prime mover and having a variable displacement hydrostaticunit, andwherein the apparatus includes a valve which is operablebetween a closed condition and an open condition to regulate'a flow ofcontrol fluid-to a motor for varying the displacement of the hydrostaticunit, a means for operating the valve from the closed condition to theopen condition in which the valve ports control fluid to the motor inresponse to a reduction in the pressure of the control fluid conductedto the valve, and an anti-stall system for reducing the pressure of thecontrol fluid conducted to the valve to effect operation of the valve tothe open condition and a porting of the reduced pressure control fluidto the motor in response to the occurrence of an impending overloadingof the prime mover.

Another object of this invention is to provide a new and improvedapparatus for controlling the operation of a hydrostatic transmissiondriven by a prime mover and having a variable displacement hydrostaticunit, I andwherein the apparatus includes a fluid motor for varying thedisplacement of the hydrostatic unit in response to variations incontrol pressure to which the motor is exposed, a pressure controllerwhich is operable to any one of a plurality of settingsto vary thecontrol pressure to which the motor is exposed with varia-- DESCRIPTIONTHE DRAWINGS The above objects andfeatures of the invention will becomemore apparent upon a consideration of the following description taken inconnection with the accompanying drawings wherein:

FIG. 1 is a schematic illustration depicting the relationship between anengine, a hydrostatic transmission, and an anti-stall system constructedin accordance with the present invention for preventing an overloadingof the engine; I v

FIG. 2 is an enlarged schematic illustration of the anti-stall system;

FIG. 3 is a sectional view of a speed control valve which is utilized inassociation with the anti-stall system, the valve being shown in an opencondition; and

FIG. 4 is a sectional view (similar to FIG. 3) of the speed controlvalve in a closed condition.

DESCRIPTION OF THE PREFERRED EMBODIMENT A known hydrostatic transmissionis illustrated schematically in FIG. 1 in association with a prime moveror engine 12. Under normal operating conditions, an output shaft 14 fromthe engine 12 drives a pump unit 18 of the hydrostatictransmission.Depending upon the position of a swashplate 19 of the pump unit 18, highpressure fluid is conducted from the pump unit through either a conduit20 or 22 to effect operation of a motor unit 24 to rotate an outputshaft 26. The operating speed of the engine 12 and rotational speed ofthe engine output shaft 14 is controlled by a known throttle arrangementindicated schematically at 30 in FIG. 1.

It is possible to vary the speed and direction of rotation of the outputshaft 26 by activating a piston and cylinder type control motor 34 tochange the annular position of the swashplate 19. When the control motor34 is extended to pivot the swashplate 19 in a counterclockwisedirection (as viewed in FIG. 1) the rotational speed of the output shaft26 is increased in a forward direction. Similarly, when the swashplate19 is pivoted in a clockwise direction from the illustrated neutralposition by retracting the control motor 34, the rotational speed of theoutput shaft 26 is increased in the reverse direction.

The control motor 34 is activated by a speed control valve 40 which isconnected with the motor by hydraulic lines or conduits 42 and 44.Movement of a control handle 48 in a forward or clockwise direction asviewed in FIG. 1 operates the valve 40 to port high pressure controlfluid through the conduit 42 to the motor 34. Similarly, when thecontrol handle 48 is moved in the reverse direction (counterclockwise asviewed in FIG. 1) high pressure fluid is ported through the conduit 44to the motor 34. The control valve 40 is advantageously of the pressureregulator type which varies the pressure of the control fluid conductedthrough the lines 42 and 44 as a direct function of the extent ofmovement of the control handle 48. The control motor 34 includes aspring box 52 which resists operation of the control motor with a forcewhich increases as the control motor is operated from the illustratedneutral position. Although the speed control valve 40 and control motor34 could be constructed in a manner similar to that disclosed in eitherU.S. Pat. No. 3,543,515 or U.S. Pat. No. 2,396,448, in accordance withone of the features of the present invention, the speed control valve iscon-' structed as shown in FIGS. 3 and 4 to provide a particularlyadvantageous relationship between the anti-stall system 58, speedcontrol valve, and control motor.

An anti-stall system 58 constructed in accordance with the presentinvention is associated with the hydrostatic transmission 10 and engine12 to prevent a stalling or overloading of the engine by the hydrostatictransmission. To accomplish this, the anti-stall system 58 effectsoperation of the control motor 34 to reduce the speed of the outputshaft 26 of the hydrostatic transmission when an impending stall oroverload condition is present. To enable the anti-stall system 58 todetect the presence of an impending stall condition, the anti-stallsystem senses the operating speed of the engine 12, the setting of thethrottle or accelerator 30,

and the input to output speed ratio of the hydrostatic transmission 10.When the combination of these factors indicates that an impending stallcondition is present, the anti-stall system 58 reduces the fluidpressure conducted from the pressure regulator or speed control valve tothe control motor 34 to enable the spring box 52 to return the controlmotor toward the illustrated neutral position. As the control motor 34returns to its neutral position, the displacement of the pump unit 18 isdecreased with a resulting decrease in the rotational speed of theoutput shaft 26 relative to the input shaft 14 to increase the input tooutput speed ratio. Of course reducing the rotational speed of theoutput shaft 26 decreases the load on the engine 12 to prevent astalling of the engine.

The anti-stall system 58 includes a first valve assembly 64 (see FIG. 2)which varies the pressure in an output conduit 68 as a function ofvariation in the operating speed of the engine 12 and the input tooutput speed ratio of the hydrostatic transmission 10. A second valveassembly 70 varies the pressure in an output conduit 74 leading to thepressure regulator valve 40 as a function of variations in the outputpressure from the valve assembly 64 and the setting of the enginethrottle 30. Therefore, the pressure of control fluid supplied to thepressure regulator valve 40 through the conduit 74 varies as a functionof three factors, that is the operating speed of the engine 12, theinput to output speed ratio of the hydrostatic transmission 10, and thesetting of the throttle 30.

Variations in the operating speed of the engine 12 are sensed bydetecting variations in a pressure differential across an'orifice 78.The upstream side of the orifice 78 is connected by a line or conduit 82with a pump 84 (FIG. 1) which is driven by the engine 12 at a speedwhich varies with variations in the operating speed of the engine. Undernormal low speed operating conditions, the output from the pump 84 issufficient to cause a pressure drop across the orifice 78. Thereforethere is a relatively high pressure in line 88 leading from the upstreamside of the orifice 78 to the first valve assembly 64 (FIG. 2) and arelatively low pressure in a line or conduit 90 leading from adownstream side of the orifice to the first and second valve assemblies64 and 70. The pressure differential across the orifice 78 and betweenthe two lines 88 and 90 varies in a known manner, with variations in theoutput speed of the engine 12.

Variations in the input to output speed ratio of the hydrostatictransmission 10, i.e., the ratio of the speed of the shaft 14 to thespeed of the shaft 26, are sensed by detecting variations in thepressure of the control fluid conducted from the pressure regulatorvalve 40 to the motor 34. Due to the influence of the spring box 52,

. an increase or decrease in the pressure of control fluidported fromthe pressure regulator valve 40 is necessary to operate the motor 34.Thus to increase the forward output speed of the hydrostatictransmission 10, the control handle 48 is moved to increase the pressureof control fluid conducted through the line 42 to a chamber 92 of thepiston and cylinder type motor 34. Increasing the pressure in thechamber 92 moves a piston 94 toward the right (as viewed in FIG. 2)against the influence of the'spring box 52 to move the swashplate 19 andincrease the effective displacement of the pump unit 18 and the speed ofthe output shaft 26 in the forward direction relative to the input shaft14.

If the control handle 48 is subsequently moved to operate the valve 40to reduce the fluid pressure in the line 42, the pressure in the chamber92 is decreased and the spring box 52 moves the piston 94 back towardthe illustrated neutral position of FIG. 2. This decreases the effectivedisplacement of the pump 18 and the output speed of the hydrostatictransmission 10. Thus, when the hydrostatic transmission is beingoperated in a forward direction, the fluid pressure in the line 42 andcylinder chamber 92 is controlled by the pressure regulator valve 40 andvaries'directly with variations in the displacement of the pump 18 andthe speed of the output shaft 26 relative to the input shaft 14. Thepressure regulator valve 40 also controls the fluid pressure in the line44 and cylinder chamber 98 to vary the output speed of the hydrostatictransmission 10 in the reverse direction. When the hydrostatictransmission 10 is being operated in the reverse direction, the fluidpressure in the line 44 and cylinder chamber 98 vary directly withvariations in the displacement of the pump 18 and the output speed ofthe hydrostatic transmission 10. r

The anti-stall system 58 prevents an overloading or stalling of theengine 12 during either forward or reverse operation. To enable theanti-stall system 58 to detect changes in the input to output speedratio during either forward or reverse operation, a rectifying valvearrangement 102 (FIG. 2) is provided for enabling the relatively highpressure in the conduit 42 to be transmitted to the anti-stall systemthrough a conduit 104 during forward operation of the hydrostatictransmission 10 and to enable a relatively high fluid pressure in theconduit 44 to be transmitted tothe anti-stall system 58 during reverseoperation of the hydrostatic transmission 10. The rectifying valvearrangement 102 includes a pair of check valves 106 and 108. Duringforward operation of the hydrostatic transmission 10 there is arelatively high fluid pressure in the conduit 42 while the conduit 44 isconnected with a drain 112 by the pressure regulator valve 40.Therefore, the high pressure in the line 42 is conducted through thecheck valve 106 to the line 104 and biases the check valve 108 to itsclosed position. Similarly, during reverse operation of the hydrostatictransmission 10, a relatively high pressure is present in the conduit 44and the conduit 42 is connected with the drain 112 by the pressureregulatOr valve 40. Therefore, the check valve 108 is open and the checkvalve 106 is biased closed to enable the high pressure in the conduit 44to be conducted through the conduit 104 to the anti-stall system 58..

The pressure in the output conduit 68 from the first valve assembly 64varies as a direct function of variations in the output speed of theengine 12 and the output speed of the hydrostatic transmission 10.Accordingly, the first valve assembly 64 has a valve spool 116 which ismoved to vary the pressure in a mixing chamber 120 as a function ofvariations in the output speed of the engine 12 and the speed of thehydrostatic transmission output shaft 26. When the valve spool 116 is inthe centered position shown in FIG. 2, the pressure in the mixingchamber 120 varies directly with variations in the operating speed ofthe engine 12. The relatively high pressure fluid from the upstream sideof the orifice 78 enters the mixing chamber 120 through an inlet 122while relatively low pressure fluid from the downstream side of theorifice 78 enters the mixing chamber 120 through an inlet 124. When theoperating speed of the engine 12 increases, the pressure differentialacross the orifice 78 increases with a resulting increase in thepressure of the control fluid supplied through the inlet 122 to themixing chamber 120. Similarly, adecrease in the operating speed of theengine 12 results in a decrease in the pressure differential across theorifice 78 with a resulting decrease in the fluid pressure in the mixingchamber 120.

The valve spool 116 is movable to vary the pressure in the mixingchamber 120 as a function of variations in the input to output speedratio of the hydrostatic transmission 10. When the speed of the inputshaft 14 is maintained constant, the pressure in the line 104 increaseswith increasing speed of the output shaft 26 and decreases withdecreasing speed of the output shaft. Thus, the pressure in the line 104varies as an inverse function of variations in the input to output speedratio.

The pressure in the line 104 is conducted through an inlet 130 to apressure chamber 132. An end surface 134 on a land 136 of the valvespool 116 is exposed to the fluid pressure in the chamber 132.Therefore, when the fluid pressure in. the conduit 104 increases, thepressure in the chamber 132 is increased to move the valve spool 116against the influence ofa biasing spring 140. This movement of the valvespool 116 increases the exposure of the mixing chamber 120 to the highpressure conduit 88 and decreases the exposure of the mixing chamber 120to the low pressure conduit 90 to thereby effect an increase in thefluid pressure in the mixing chamber when the pressure in the conduit104 is increased with an increase in the output speed of the hydrostatictransmission 10.

. fluid pressure supplied to the pressure regulatorvalve 40 as afunction of the output from the first valve assembly 64 and as afunction of the setting of the throttle 30. The second valve assemblyincludes a valve spool 144 which is disposed in a chamber 146 connectedin fluid communication with the low pressure line through an inlet port150. The valve chamber 146 is connected with drain through an outletport 152.

A diaphragm type motor 156 moves the valve spool 144 as a function ofvariations in the pressure in the conduit 68. Movement of the valvespool 144 by the motor 156 varies the extent to which the valve chamber146 is connected with'the fluid supply line 90 through the inlet 150 andwith drain through the outlet 152.

The motor 156 includes a diaphragm 166 which is con- .120 of the firstvalve assembly 64. The lower side of the diaphragm 166 is exposed to therelatively constant fluid pressure in the'supply line 90.

- An increase in the fluid pressure in the mixing chamber of the firstvalve assembly 64 results in the valve spool 144 of the second valveassembly 70 being moved downwardly (as viewed in FIG. 2) by the motor156. This movement increases the extent to which the outlet conduit 74and pressure regulator valve 40 are connected with the fluid supply line90 and decreases the extent to which they are connected to drain throughthe opening 152. This results in an increase in the pressure of thecontrol fluid supplied though the conduit 74 to the pressure regulatorvalve 40. Similarly, a decrease in the pressure conducted to the fluidmotor 156 through the conduit 68 results in upward movement of the valvespool 144 to decrease the pressure of the control fluid supplied throughthe conduit 74 to the pressure regulator valve 40.

In addition to varying the pressure of the control fluid supplied to thepressure regulator valve 40 as a function of variations in the outputfrom the first valve assembly 64, the second valve assembly 70 variesthe pressure of the control fluid as a function of the setting of thethrottle 30. When the throttle 30 is set for a relatively high speed,the fluid pressure in the mixing chamber 120 of the first valve assembly64 increases due to an increase in the pressure from the engine drivenpump 84. Similarly, when the throttle 30 is set for relatively low speedoperation of the engine 12, the fluid pressure in the mixing chamber 120of the first valve assembly 64 will decrease as the output pressure fromthe engine driven pump 84 decreases. Therefore, when the throttle 30 isset for high speed operation of the engine 12, a relatively large forceis applied by a spring 170 against one end of the valve spool 144 to atleast partially offset the increasing pressure applied to the diaphragm166 at the opposite end of the valve spool 144. Similarly, when thethrottle 30 is set for relatively low speed operation of the engine 12,the spring 170 applies a reduced force against the end of the valvespool 144.

To provide for variation in the force applied by the spring 170 againstthe valve spool 144, movement of the throttle 30 moves a threaded member174 axially along a passage 146 to vary the extent to which the spring170 is compressed against the valve spool 144. Thus, when the throttle30 is set for relatively high speed operation of the engine 12, thethreaded member 174 is turned into the passage 146 to increase thepressure applied by the spring 170 against the valve spool 144. Ofcourse, when the throttle 30 is moved to set the engine for relativelylow speed operation the threaded member 174 is moved outwardly todecrease the extent to which the spring 170 is compressed against thevalve member 144. Thus, the second valve assembly 70 is operable to varythe output from the first valve assembly 64 as a function of changes inthe setting of the throttle 30.

When the hydrostatic transmission 10 is being driven in a forwarddirection by the engine 12 under normal operating conditions, the firstand second valve assemblies 64 and 70 maintain the pressure in thesupply line 74 to the pressure regulator valve 40 at a level which isjust slightly greater than the level of the pressure in the line 42leading from the valve 40 to the control motor 34. WHen an impendingstall condition is encountered, the pressure in the line 74 to thepressure regulator valve 40 is reduced to effect a reduction in thefluid pressure in the line 42 to the control motor 34. Of course,reducing the fluid pressure in the conduit 42 enables the spring box 52to operate the motor 34 to move the swashplate 19 of the pump unit 18toward the neutral condition to reduce the output speed of thehydrostatic transmission 10. Reducing the output speed of thehydrostatic transmission 10 reduces the load on the engine 12 to preventa stalling of the engine. Of course, during reverse operation of thehydrostatic transmission 10 the fluid pressure in the conduit 44 isreduced during an impending stall condition to reduce the speed ofoperation of the hydrostatic transmission in the reverse direction.

To enable the anti-stall system 58 to quickly effect a reduction in theoutput speed of a hydrostatic transmission in the event of an impendingstalling of the engine 12, the valve assemblies 64 and maintain thepressure of the control fluid supplied to the pressure regula tor valve40 through the conduit 74 at a level which is just slightly greater thanthe fluid pressure in the conduit 42 or 44. Thus, when the motor 34 isoperated to a relatively small extent from the illustrated neutralposition against the influence of a spring box 52, the pressure in thesupply conduit 74 for the pressure regulator valve 40 is relatively lowand is only slightly greater than the fluid pressure in the conduit 42.Similarly. when the pressure regulator valve 40 is operated in an extentsufficient to effect operation of the control motor in a forwarddirection to a relatively large extent, the pressure in the supplyconduit 74 is relatively large and is still only slightly greater thanthe relatively large pressure which will be present in the conduit 42extending between the pressure regulator valve 40 and control motor 34.Of course, during operation of the hydrostatic transmission 10 in thereverse direction, the pressure in the supply conduit 74 varies with andis slightly greater than the pressure conducted to the control motor 34fromthe pressure regulator valve 40 through the conduit 44.

The foregoing relationship between the pressure in the supply conduit 74and the pressure ported from the valve 40 to the control motor 34 ismaintained in the absence of an impending stall regardless of theposition to which the engine throttle lever 30 is set. This is becausethe second valve assembly 70 compensates for changes in operating speeddue to variations in throttle setting.

The pressure regulator or speed control valve 40 is operated from aclosed condition to an open condition in response to a reduction in thefluid pressure in the conduit 74 by the anti-stall system 58. Operationof the speed control valve 40 to the open condition exposes the controlmotor 34 to the reduced pressure in the conduit 74 to enable the springbox 52 to effect a reduction in the displacement of the pump 18 and a reduction in the load on the engine 12. The valve 40 in cludes a valvespool 200 (see FIGS. 3 and 4) which is slidably disposed in a generallycylindrical valve chamber 204 in a housing 206. The valve chamber 204 isconnected in continuous fluid communication with the anti-stall system58 by the conduit 74 and with the motor 34 by the conduits 42 and 44 inthe manner illustrated schematically in FIGS. 1 and 2. The valve chamber204 is also connected with the drain 112 by conduit 207.

Movement of the control handle 48 from the neutral position (illustratedin dashed lines in FIG. 3) to a forward actuated position (illustratedin solid lines in FIG. 3) moves the valve spool 200 from a centered orclosed position within the housing 206 toward the left to the actuatedor open position illustrated in FIG. ,3. This movement of the valvespool 200 ports control fluid from the conduit '74 to the conduit 42 andmotor 34. Thus, high pressure fluid from the anti-stall system 58 entersthe valve chamber 204 and passes through an annular passage between twocircular lands 208 and 210 on the open valve spool 200 to the conduit42. This high pressure fluid also flows through a radially extendingpassage 214 formed at the land 208 to a pressure chamber 216.

While high pressure fluid is being directed from the supply line 74through the conduit 42 to one end of the fluid motor 34, the oppositeend of the fluid motor is connected with the drain 112. Thus, theconduit 44 is connected through an annular passage between lands 2l0and220 on the valve spool 200 with an entrance 221 to the drain passage207. It should be noted that a pressure chamber 222'is connected withdrain by a radially extending passage 228 extending through the circularland 210. r

I As the motor 34 is operated against the influence of the spring box52, the pressure in the conduit 42 increases. This increase in fluidpressure is transmitted throughthe passage 104 to the anti-stall system58 which, in the absence of an impending stalling of the engine 12,increases the fluid pressure in the conduit 74 to maintain the fluidpressure in the conduit 74 slightly above the fluid pressure conductedfrom thespeed control valve 40 to the control motor 34. The increase influid pressure in the conduit 42 is transmitted through the passage 214to the pressure chamber 216 which is located between a piston element230 and the valve spool 200. As the pressure in the chamber increases,the valve spool 220 slides axially along the support rod 232 from theopen position of FIG. 3 toward the closed position of FIG. 4 against theinfluence of a biasing spring 234.

When the valve spool 200 reaches the closed position shown in FIG. 4,the land 208 is midway of an annular recess 238 in the housing 206. Thepassage 42 is then connected through the recess 238 in fluidcommunication with both the supply conduit 74 and with. an entrance242'to the drain passage 207. The land 208 on the closed valve spool 200substantially blocks fluid flow to and from the conduit 42 by providinga modulating action between the conduit 42, control fluid supply conduit74 and the drain conduit 207. This modu- Iating action maintains thefluid motor 34 in an operated position which corresponds-t the extentwhich the handle 48 is moved from the neutral position. However, whenthe valve spool 200 is in the closed position, the conduit 42 and motor34 are connected in fluid communication with both the drain 112 and thecontrol fluid supply conduit 74' to facilitate rapid response of themotor 34 to operation of either the anti-stall system 58 or controlvalve 40.

Movement of the control handle 48 from the neutral position shown indashed lines in FIG. 3 results in operation of control motor 34 to anextent which is proportional to the distance which the handle is moved.This is because the fluid pressure in the conduit 42 effects movement ofthe piston 94 against the spring box 52 and of the valve spool 200against the biasing spring 234. The further the handle 48 is moved in aforward direction from the neutral position, the greater is the extentto which the spring 234 is compressed and the greater is the fluidpressure which must be built-upin the pressure chamber 216 to move thevalve spool 200 from the open position to the closed position. Of

course, the fluid pressure in the chamber 216 is directly proportionalto the fluid pressure in the conduit 42. Therefore, the greater thefluid pressure in the chamber 216, the greater the pressure in thechamber 92 of the motor 34 and the distance through which the piston 94is moved against the influence of the spring box 52. The generalrelationship between the extent to which the handle 48 is moved and theextent to which the control motor 34 is operated is substantially thesame as is disclosed in U.S. Pat. No. 3,540,220 which is included hereinby this reference thereto.

When the valve spool 200 is in the closed position of FIG. 4 and thefluid pressure conducted through the conduit 74 is decreased due tooperation of the antistall system 58, this decrease in fluid pressure isimmediately communicated around the land 208 to the motor 34 throughconduit 42 and to the pressure chamber 216 through th passage 214. Thedecrease in the fluid pressure in the chamber 216 enables the spring 234to move'the valve spool 200 toward the left from the closed positionshown in FIG. 4 toward the position shown in FIG. 3. This movement ofthe valve spool 200 increases the communication of the conduit 44 withdrain and of the conduit 42 with the fluid pressure supply line 74.Since the fluid pressure in the supply line 74 has been reduced byoperation of the anti-stall system 58, the spring box 52 returns themotor 34 toward its neutral position, shown in FIG. 2. The valve spoolremains in open position until the fluid pressure in the supply conduit74 increases sufficiently to again move the valve spool against theinfluence of the biasing spring 234 to the position shown in FIG. 4. Ofcourse at this time the fluid pressure in the conduit 42 is sufflcientto operate the motor 34 against the influence of the spring box 52 to aposition corresponding to the extent of actuation of the control handle48.

When the control handle 48 is operated in the reverse direction, thevalve spool is moved toward the right hand (as viewed in FIG. 3) toconnect the conduit 44 with the supply line 74 and to connect theconduit 42 with drain 112 through the passage 207. As the pressure inthefluid conduit 44 builds up, it is transmitted through the radialpassage 228 to the pressure chamber 222 to move the valve spool to thecentered modulating position of FIG. 4 against the influence of abiasing spring 250. Of course, upon the occurrence of an impending stallthe biasing spring 250 moves the valve spool 200 back toward the rightto connect the conduit 44 to the reduced pressure in the supply conduit74. Although the speciflc valve 40 shown in FIGS. 3 and 4 is preferred,it should be understood that other known valve constructions could beutilized with an anti-stall system 58 constructed in accordance with thepresent invention.

- From the foregoing description it is apparent that the anti-stallsystem 58 includes a first valve assembly 64 which cooperates with theorifice 78 and control motor 34 to detect variations in the operatingspeed of the engine 12 and the input to output speed ratio of thehydrostatic transmission 10. The output pressure from the first valveassembly 64 varies as a function of these two variables. This outputpressure is related to throttle settings by the second valve assemblywhich provides a source of control fluid for the valve 40. Therefore,the pressure of the control fluid supplied to the valve 40 varies as afunction of variations in engine operating speed, input to output speedratio of the hydrostatic transmission 10, and setting of the throttle30.

Toenable the anti-stall system 58 to respond quickly toan impendingoverloading of the engine 12, the two valve assemblies 64 and 70maintain the pressure of the control fluid supplied to the valve 40through the con duit 74 at a level which is slightly greater than thepressure in the higher pressure one of the two conduits 42 and 44. Thus,during forward operation and in the absence of an impending stall, thepressure of the control fluid in the conduit 74 increases and decreasesas the fluid pressure in the conduit 42 increases and decreases.Although the fluid pressure in the conduit 74 varies with variations inthe fluid pressure in the conduit 42 during forward operation, the fluidpressure in the conduit 74 is maintained at a level which is slightlygreater than the fluid pressure in the conduit 42 until an impendingstall condition is encountered.

When an impending stall condition is encountered, the operating speed ofthe engine 12 and pump 84 decrease with a resulting decrease in thefluid pressure at the inlet 122 to the mixing chamber 120. This resultsin a decrease in the pressure communicated through conduit 68 to thediaphragm motor 156 and upward movement of the valve spool 144. As thevalve spool 144 moves upwardly, the fluid pressure in the conduit 74decreases. This decrease in fluid pressure is communicated through thevalve 40 and one of the conduits 42 and 44 to the motor 34. The springbox 52 then operates the motor 34 to decrease the speed of the outputshaft 26 to reduce the load on the engine 12.

Having described a specific preferred embodiment of the invention, thefollowing is claimed:

1. Apparatus for controlling the operation of a hydrostatic transmissionhaving a variable displacement hydrostatic unit connected with a primemover having a selectively settable throttle, said apparatus comprisingmotor means for varying the displacement of the hydrostatic unit to varythe output speed of the hydrostatic transmission, means for providing afirst signal which varies with variations in the operating speed of theprime mover, means for providing a second signal which varies withvariations in the setting of the throttle, means for providing a thirdsignal which varies as a function of variations in the displacement ofthe hydrostatic unit, and anti-stall means responsive to said first,second and third signals for effecting operation of said motor means tovary the displacement of the hydrostatic unit to reduce the output speedof the hydrostatic transmission upon the occurrence of an impendingoverloading of the prime mover.

2. An apparatus as set forth in claim 1 further including control meansfor porting fluid to said motor means at a pressure corresponding to adesired displacement of the hydrostatic unit in the absence of animpending overloading of the prime mover, said anti-stall meansincluding meansfor porting fluid to said control means at a pressurewhich is greater than and which varies as a function of variations inthe pressure of the fluid ported from said control means to said motormeans.

3. An apparatus as set forth in claim said anti-stall whereinsaidanti-stall means includes means for reducing the fluid pressuresported to said control means and to said motor means in response to theoccurrence of an impending overloading of the prime mover.

4. An apparatus as set forth in claim 2 wherein said means for providinga third signal includes means for sensing the fluid pressure ported fromsaid control means to said motor means.

5. An apparatus as set forth in claim 1 wherein said anti-stall meansincludes valve means for porting fluid at a pressure which varies as afunction of said first, second and third signals, said means forproviding a first signal including means for conducting to said valvemeans fluid at a pressure which varies as a function of variations inthe operating speed of the prime mover,

said means for providing a second signal including means for applying tosaid valve means a pressure which varies with variations in the throttlesetting, said means for providing a third signal includes means forconducting to said valve means fluid at a pressure which varies as afunction of variations in the displacement of the hydrostatic unit. A

6. Apparatus as set forth in claim 1 further including control valvemeans operable between a closed condition and an open conditionconnecting said motor means in fluid communication with said anti-stallmeans, said valve means including means for operating said valve meansfrom the closed condition to the open condition in response to operationof said anti-stall means upon the occurrence of an impending overloadingof the prime mover.

7. An apparatus as set forth in claim 1 further including speed controlvalve means operable from a closed condition to an open condition toport fluid under pressure to said motor means to effect operation ofsaid motor means and a variation in the displacement of the hydrostaticunit, and manually actuatable means foreffecting operation of said speedcontrol valve means between the open and closed conditions to effect adesired variation in the displacement of the hydrostatic unit, saidspeed control valve means including means for effecting operation ofsaid speed control valve means from the closed condition to the opencondition in response to operation of said anti-stall means upon theoccurrence of an impending overloading of the prime mover.

8. Apparatus for controlling the operation of a hydrostatic transmissiondriven by a prime mover,said apparatus comprising motor means forvarying the input to output speed ratio of the hydrostatic transmissionin response to variations in pressure of a control fluid supplied tosaid motor means, control means for varying the pressure of the controlfluid supplied to said motor means, means for conducting fluid to saidcontrol means from a source of control fluid, and anti-stall means forreducing the pressure of control fluid conducted to said control meansand to said motor means to effect operation of said motor means toreduce the output speed of the hydrostatic transmission in response tothe occurrence of an impending overloading of the prime mover and formaintaining the pressure of the control fluid supplied to said controlmeans at a pressure which is greater than the pressure of the controlfluid supplied to said motor means and varying the pressure of thecontrol fluid supplied to said control means as a function of variationsin the pressure of the control fluid supplied to said motor means in theabsence of an impending overloading of the prime mover.

9. An apparatus as set forth in claim 8 wherein said anti-stall meansincludes means for increasing the pressure of the control fluid suppliedto said control means in response to a sensing of an increase in thepressure of the control fluid supplied to said motor means in theabsence of an impending overloading of the prime mover.

10. An apparatus as set forth in claim 8 further including means forvarying the pressure of the control fluid supplied to said control meansas a function of variations in the output speed of the prime mover.

11. An apparatus as set forth in claim 8 wherein said anti-stall meansincludes first conduit means for conducting fluid at a first pressure,second conduit means for conducting fluid at a second pressure, meansfor varying at least one of said first and second pressures as afunction of variations in the operating speed of the prime mover, meansdefining a mixing chamber connected in fluid communication with saidfirst and second conduit means, valve means for varying the rate of flowof fluid from said first and second conduit means into said mixingchamber to vary the fluid pressure in said mixing chamber, actuatormeans for effecting operation of said valve means as a function ofvariations in the pressure of the control fluid supplied to said motormeans, and means for supplying control fluid to said means forconducting control fluid to said control means at a third pressure whichvaries as a function of variations in the fluid pressure in said mixingchamber.

12. An apparatus as set forth in claim 11 wherein said means forsupplying control fluid at a third pressure includes pressure regulatormeans connected in fluid communication with said means for conductingcontrol fluid to said control means and with said mixing chamber forvarying said third pressure as a function of variations in the fluidpressure in said mixing chamber and as a function of variations in thesetting of a throttle for the prime mover, said pressure regulator meansincluding a valve member, means for applying to said valve member aforce which varies as a function of variations in the fluid pressure insaid mixing chamber, and means for applying to said valve member a forcewhich varies as a function of variations in the setting of the throttlefor the prime mover.

13. An apparatus as set forth in claim 8 wherein said motor means isoperable in a first direction from an initial position to effectoperation of the hydrostatic transmission in a forward direction and isoperable in a second direction from the initial position to effectoperation of the hydrostatic transmission in a reverse direction, saidapparatus further including first conduit means for conducting controlfluid from said control means to said motor means to effect operation ofsaid motor means in the first direction, second conduit means forconducting control fluid from said control means to said motor means toeffect operation of said motor means in the second direction, and valvemeans for enabling the fluid pressure in said first conduit means to betransmitted to said anti-stall means during operation of said motormeans in the first direction and for enabling the fluid pressure in saidsecond conduit means to be transmitted to said anti-stall means duringoperation of said motor means in the second direction.

14. An apparatus as set forth in claim 13 wherein said anti-stall meansincludes means for increasing the pressure of control fluid conducted tosaid control means in response to an increase in the fluid pressuretransmitted to said anti-stall means by way of said valve means duringoperation of said motor means in one of said first and second directionsand for decreasing the pressure of control fluid conducted to saidcontrol means in response to a decrease in the fluid pressuretransmitted to said anti-stall means by way of said valve means duringoperation of said motor means in one of said first andsecond directions.

15. An apparatus as set forth in claim 8 wherein said control meansincludes a valve member movable between a closed position and an openposition in which control fluid can flow between the source of controlfluid and said motor means, and means for moving said valve member fromthe closed position to the open powith drain and means for moving saidvalve member from the closed position to the open position in responseto a reduction in pressure of the control fluid conducted to saidcontrol means.

17. Apparatus comprising a variable diplacement fluid pump, fluid motormeans for varying the displacement of said pump, control valve means fordirecting fluid pressure to said fluid motor means and for effectingoperation of said fluid motor means by varying the fluid pressuredirected to said fluid motor means, means for directing fluid pressureto said valve means, and anti-stall means forvarying the fluid pressuredirected to said valve means with variations in the fluid pressuredirected to said fluid motor means to maintain the fluid pressuredirected to said valve means slightly above the pressure directed tosaid fluid motor means for all displacements of said pump in the absenceof an impending stall condition, said anti-stall means including meansfor sensing an impending stall condition and for lowering the pressuredirected to said valve means in response to the sensing of an impendingstall condition to immediately effect operation of said motor means toreduce the displacement of said pump.

18. Apparatus as set forth in claim 17 wherein said anti-stall meansincludes means for sensing variations in the fluid pressure directed tosaid motor means and pressure regulator means for increasing thepressure of the fluid directed to said valve means in response to anincrease in the fluid pressure directed to said motor means and fordecreasing the pressure of the fluid directed to said valve means inresponse to a decrease in the fluid pressure directed to said motormeans.

19. Apparatus as set forth in claim 17 wherein said valve means includesmeans for varying the pressure of the fluid directed to said fluid motormeans in response to variations in the fluid pressure directed to saidvalve means.

20. Apparatus as set forth in claim 17 wherein said valve means includesa valve spool and means for moving said valve spool from a closedposition at least partially blocking fluid flow to said fluid motormeans to an open position in response to a lowering of the fluidpressure directed to said valve means.

21. Apparatus for controlling the operation of a hydrostatictransmission driven by a prime mover and having a variable displacementhydrostatic unit, said apparatus comprising motor means for varying thedisplacement of the hydrostatic unit in response to variations in thepressure of control fluid supplied to said motor means, valve meansoperable between a closed condition and an open condition to regulate aflow of control fluid to said motor means, means for conducting controlfluid to said valve means from a source of control fluid, means foroperating said valve means from the closed condition to the opencondition in which said valve means ports the control fluid to saidmotor means in response to a reduction in the pressure of the controlfluid conducted to said valve means, and anti-stall means for reducingthe pressure of the control fluid conducted to said valve means toeffect operation of said valve means to the open condition and a portingof relatively low pressure control fluid to said motor means in responseto the occurrence of an impending overloading of the prime mover.

22. An apparatus as set forth in claim 21 further including manuallyactuatable means movable from an initial position to any one of aplurality of actuated positions spaced different distances from saidinitial posi tion, means for operating said valve means from the closedcondition to the open condition in response to movement of said manuallyactuatable means from the initial position to any one of said actuatedpositions to port control fluid to said motor means, and means foroperating said valve means from the open condition to the closedcondition in response to operation of said motor means to an extentwhich is a function of the extent of movement of said manuallyactuatable means from the initial position in the absence of animpending overloading of the prime mover.

23. An apparatus as set forth in claim 22 wherein said means foroperating said valve means from the open condition to the closedcondition in response to operation of said motor means includes pressureresponsive means for operating said valve means to the closed conditionin response to an increase in the pressure of the control fluidconducted to said motor means to a pressure corresponding to the extentof movement of said manually actuatable means from the initial position,said anti-stall means being effective to prevent an increase in thepressure of the control fluid conducted to said motor means to apressure corresponding to the extent of movement of said manuallyactuatable means from'the initial position in'response to an impendingoverloading of the prime mover.

24; An apparatus as set forth inclaim 21 wherein said anti-stall meansincludes means for maintaining the pressure of the control fluidconducted to said valve means at a pressure which is greater than thepressure of the control fluid supplied to said motor means and forvarying the pressure of the control fluid supplied to said valve meansas a function of variations in the pressure of the control fluidsupplied to said motor means.

25. An apparatus for controlling the operation of a hydrostatictransmission driven by a prime mover and having a variable displacementhydrostatic unit, said apparatus comprising fluid motor means forvarying the displacement of the hydrostatic unit in response tovariations in control fluid pressure to which said motor means isexposed, pressure controller means operable to anyone of a plurality ofsettings to vary the control fluid pressure to which said motor means isexposed with variations in the setting of said pressure controllermeans, and anti-stall means connected in continuous fluid communicationwith said motor means through said pressure controller means fordecreasing the pressure to which said motor means is exposed in responseto the occurrence of an impending overloading of the prime mvoer.

26. An apparatus for controlling the operation of a hydrostatictransmission driven by a prime mover and having a variable displacementhydrostatic unit, said apparatus comprising fluid motor means forvarying the displacement of the hydrostatic unit in response tovariations in control fluid pressure to which said motor means isexposed, pressure controller means operable to anyone of a plurality ofsettings to vary the control fluid pressure to which said motor means isexposed with variations in the setting of said pressure controllermeans, and anti-stallmeans connected in fluid communication with saidmotor means through said pressure controller means for decreasing thepressure to which said motor means is exposed in response to theoccurrence of an impending overloading of the prime mover, said pressurecontroller means including valve means operable between a firstcondition varying the control fluid pressure to which said motor meansis exposed and a second condition in which the control fluid pressure towhich said motor means is exposed is maintained substantially constantand passage means for connecting said anti-stall means in fluidcommunication with said motor means when said valve means is in eitherthe first condition or the second condition.

27. An apparatus for controlling the operation of a hydrostatictransmission driven by a prime mover and having a variable displacementhydrostatic unit, said apparatus comprising fluid motor means forvarying the displacement of the hydrostatic unit in response tovariations in control fluid pressure to which said motor means isexposed, pressure controller means operable to anyone of a plurality ofsettings to vary the control fluid pressure to which said motor means isexposed with variations in the setting of said pressure controllermeans, anti-stall means connected in fluid communication with said motormeans through said pressure controller means for decreasing the pressureto which said motor means is exposed in response to the occurrence of animpending overloading of the prime mover, means for providing a firstsignal which varies with variations in the operating speed of the primemover, means for providing a second signal which varies with variationsin the setting of a throttle for the prime mover, and means forproviding a third signal which varies as a function of variations in thedisplacement of the hydrostatic unit, said anti-stall means includingmeans responsive to said first, second and third signals for reducingthe control fluid pressure to which said motor means is exposed upon theoccurrence of an impending overloading of the prime mover.

28. An apparatus for controlling the operation of a hydrostatictransmission driven by a prime mover and having a variable displacementhydrostatic unit, said apparatus comprising fluid motor means forvarying the displacement of the hydrostatic unit in response tovariations in control fluid pressure to which said motor means isexposed, pressure controller means operable to anyone of a plurality ofsettings to vary the control fluid pressure to which said motor means isexposed with variations in the setting of said pressure controllermeans, and anti-stall means connected in fluid communication with saidmotor means through said pressure controller means for decreasing thepressure to which said motor means is exposed in response to theoccurrence of an impending overloading of the prime mover, saidanti-stall means including means for maintaining the pressure of controlfluid conducted to said pressure controller slightly above the controlfluid pressure to which said motor means is exposed for alldisplacements of said hydrostatic unit in the absence of an impendingoverloading of the prime mover.

29. An apparatus for controlling the operation of a hydrostatictransmission driven by a prime mover and having a variable displacementhydrostatic unit, said apparatus comprising fluid motor means for varyinvarying displacement of the hydrostatic unit in response to variationsin control fluid pressure to which said motor means is exposed, pressurecontroller means operable to any one of a plurality of settings to varythe control fluid pressure to which said motor means is exposed withvariations in the setting of said pressure controller means, andanti-stall means connected in fluid communication with said motor meansthrough said pressure controller means for decreasing the pressure towhich said motor means is exposed in response to the occurrence of animpending overloading of the prime mover, said pressure controller meansincluding valve means for varying the control fluid pressure to whichsaid motor means is exposed, said valve means being operable between afirst condition varying the control fluid pressure to which said motormeans is exposed and a second condition in which the control fluidpressure to which said motor means is exposed is maintainedsubstantially constant, said apparatus further including fluid pressurefeedback means for urging operation of said valve means from the firstcondition to the second condition under the influence of a fluidpressure force which varies as a function of variations in the controlfluid pressure to which said motor means is exposed, and means forurging said valve means against operation to the second condition whicha second force with corresponds to a setting of said pressure controllermeans.

1. Apparatus for controlling the operation of a hydrostatic transmissionhaving a variable displacement hydrostatic unit connected with a primemover having a selectively settable throttle, said apparatus comprisingmotor means for varying the displacement of the hydrostatic unit to varythe output speed of the hydrostatic transmission, means for providing afirst signal which varies with variations in the operating speed of theprime mover, means for providing a second signal which varies withvariations in the setting of the throttle, means for providing a thirdsignal which varies as a function of variations in the displacement ofthe hydrostatic unit, and anti-stall means responsive to said first,second and third signals for effecting operation of said motor means tovary the displacement of the hydrostatic unit to reduce the output speedof the hydrostatic transmission upon the occurrence of an impendingoverloading of the prime mover.
 2. An apparatus as set forth in claim 1further including control means for porting fluid to said motor means ata pressure corresponding to a desired displacement of the hydrostaticunit in the absence of an impending overloading of the prime mover, saidanti-stall means including means for porting fluid to said control meansat a pressure which is greater than and which varies as a function ofvariations in the pressure of the fluid ported from said control meansto said motor means.
 3. An apparatus as set forth in claim saidanti-stall wherein saidanti-stall means includes means for reducing thefluid pressures ported to said control means and to said motor means inresponse to the occurrence of an impending overloading of the primemover.
 4. An apparatus as set forth in claim 2 wherein sAid means forproviding a third signal includes means for sensing the fluid pressureported from said control means to said motor means.
 5. An apparatus asset forth in claim 1 wherein said anti-stall means includes valve meansfor porting fluid at a pressure which varies as a function of saidfirst, second and third signals, said means for providing a first signalincluding means for conducting to said valve means fluid at a pressurewhich varies as a function of variations in the operating speed of theprime mover, said means for providing a second signal including meansfor applying to said valve means a pressure which varies with variationsin the throttle setting, said means for providing a third signalincludes means for conducting to said valve means fluid at a pressurewhich varies as a function of variations in the displacement of thehydrostatic unit.
 6. Apparatus as set forth in claim 1 further includingcontrol valve means operable between a closed condition and an opencondition connecting said motor means in fluid communication with saidanti-stall means, said valve means including means for operating saidvalve means from the closed condition to the open condition in responseto operation of said anti-stall means upon the occurrence of animpending overloading of the prime mover.
 7. An apparatus as set forthin claim 1 further including speed control valve means operable from aclosed condition to an open condition to port fluid under pressure tosaid motor means to effect operation of said motor means and a variationin the displacement of the hydrostatic unit, and manually actuatablemeans for effecting operation of said speed control valve means betweenthe open and closed conditions to effect a desired variation in thedisplacement of the hydrostatic unit, said speed control valve meansincluding means for effecting operation of said speed control valvemeans from the closed condition to the open condition in response tooperation of said anti-stall means upon the occurrence of an impendingoverloading of the prime mover.
 8. Apparatus for controlling theoperation of a hydrostatic transmission driven by a prime mover, saidapparatus comprising motor means for varying the input to output speedratio of the hydrostatic transmission in response to variations inpressure of a control fluid supplied to said motor means, control meansfor varying the pressure of the control fluid supplied to said motormeans, means for conducting fluid to said control means from a source ofcontrol fluid, and anti-stall means for reducing the pressure of controlfluid conducted to said control means and to said motor means to effectoperation of said motor means to reduce the output speed of thehydrostatic transmission in response to the occurrence of an impendingoverloading of the prime mover and for maintaining the pressure of thecontrol fluid supplied to said control means at a pressure which isgreater than the pressure of the control fluid supplied to said motormeans and varying the pressure of the control fluid supplied to saidcontrol means as a function of variations in the pressure of the controlfluid supplied to said motor means in the absence of an impendingoverloading of the prime mover.
 9. An apparatus as set forth in claim 8wherein said anti-stall means includes means for increasing the pressureof the control fluid supplied to said control means in response to asensing of an increase in the pressure of the control fluid supplied tosaid motor means in the absence of an impending overloading of the primemover.
 10. An apparatus as set forth in claim 8 further including meansfor varying the pressure of the control fluid supplied to said controlmeans as a function of variations in the output speed of the primemover.
 11. An apparatus as set forth in claim 8 wherein said anti-stallmeans includes first conduit means for conducting fluid at a firstpressure, second conduit means for conducting fluid at a secondpressure, means for varying at least one of said first and secondpressures as a function of variations in the operating speed of theprime mover, means defining a mixing chamber connected in fluidcommunication with said first and second conduit means, valve means forvarying the rate of flow of fluid from said first and second conduitmeans into said mixing chamber to vary the fluid pressure in said mixingchamber, actuator means for effecting operation of said valve means as afunction of variations in the pressure of the control fluid supplied tosaid motor means, and means for supplying control fluid to said meansfor conducting control fluid to said control means at a third pressurewhich varies as a function of variations in the fluid pressure in saidmixing chamber.
 12. An apparatus as set forth in claim 11 wherein saidmeans for supplying control fluid at a third pressure includes pressureregulator means connected in fluid communication with said means forconducting control fluid to said control means and with said mixingchamber for varying said third pressure as a function of variations inthe fluid pressure in said mixing chamber and as a function ofvariations in the setting of a throttle for the prime mover, saidpressure regulator means including a valve member, means for applying tosaid valve member a force which varies as a function of variations inthe fluid pressure in said mixing chamber, and means for applying tosaid valve member a force which varies as a function of variations inthe setting of the throttle for the prime mover.
 13. An apparatus as setforth in claim 8 wherein said motor means is operable in a firstdirection from an initial position to effect operation of thehydrostatic transmission in a forward direction and is operable in asecond direction from the initial position to effect operation of thehydrostatic transmission in a reverse direction, said apparatus furtherincluding first conduit means for conducting control fluid from saidcontrol means to said motor means to effect operation of said motormeans in the first direction, second conduit means for conductingcontrol fluid from said control means to said motor means to effectoperation of said motor means in the second direction, and valve meansfor enabling the fluid pressure in said first conduit means to betransmitted to said anti-stall means during operation of said motormeans in the first direction and for enabling the fluid pressure in saidsecond conduit means to be transmitted to said anti-stall means duringoperation of said motor means in the second direction.
 14. An apparatusas set forth in claim 13 wherein said anti-stall means includes meansfor increasing the pressure of control fluid conducted to said controlmeans in response to an increase in the fluid pressure transmitted tosaid anti-stall means by way of said valve means during operation ofsaid motor means in one of said first and second directions and fordecreasing the pressure of control fluid conducted to said control meansin response to a decrease in the fluid pressure transmitted to saidanti-stall means by way of said valve means during operation of saidmotor means in one of said first and second directions.
 15. An apparatusas set forth in claim 8 wherein said control means includes a valvemember movable between a closed position and an open position in whichcontrol fluid can flow between the source of control fluid and saidmotor means, and means for moving said valve member from the closedposition to the open position in response to a reduction in the pressureof control fluid conducted to said control means.
 16. An apparatus asset forth in claim 8 wherein said control means includes a valve membermovable between a closed position and an open position in which saidmotor means is connected in fluid communication with drain and means formoving said valve member from the closed position to the open positionin response to a reduction in pressure of the control fluid conducted tosaid control means.
 17. Apparatus comprising a variable diplacementfluid pump, fluid motor means for varying the displacement of said pump,control valve means for directing fluid pressure to said fluid motormeans and for effecting operation of said fluid motor means by varyingthe fluid pressure directed to said fluid motor means, means fordirecting fluid pressure to said valve means, and anti-stall means forvarying the fluid pressure directed to said valve means with variationsin the fluid pressure directed to said fluid motor means to maintain thefluid pressure directed to said valve means slightly above the pressuredirected to said fluid motor means for all displacements of said pump inthe absence of an impending stall condition, said anti-stall meansincluding means for sensing an impending stall condition and forlowering the pressure directed to said valve means in response to thesensing of an impending stall condition to immediately effect operationof said motor means to reduce the displacement of said pump. 18.Apparatus as set forth in claim 17 wherein said anti-stall meansincludes means for sensing variations in the fluid pressure directed tosaid motor means and pressure regulator means for increasing thepressure of the fluid directed to said valve means in response to anincrease in the fluid pressure directed to said motor means and fordecreasing the pressure of the fluid directed to said valve means inresponse to a decrease in the fluid pressure directed to said motormeans.
 19. Apparatus as set forth in claim 17 wherein said valve meansincludes means for varying the pressure of the fluid directed to saidfluid motor means in response to variations in the fluid pressuredirected to said valve means.
 20. Apparatus as set forth in claim 17wherein said valve means includes a valve spool and means for movingsaid valve spool from a closed position at least partially blockingfluid flow to said fluid motor means to an open position in response toa lowering of the fluid pressure directed to said valve means. 21.Apparatus for controlling the operation of a hydrostatic transmissiondriven by a prime mover and having a variable displacement hydrostaticunit, said apparatus comprising motor means for varying the displacementof the hydrostatic unit in response to variations in the pressure ofcontrol fluid supplied to said motor means, valve means operable betweena closed condition and an open condition to regulate a flow of controlfluid to said motor means, means for conducting control fluid to saidvalve means from a source of control fluid, means for operating saidvalve means from the closed condition to the open condition in whichsaid valve means ports the control fluid to said motor means in responseto a reduction in the pressure of the control fluid conducted to saidvalve means, and anti-stall means for reducing the pressure of thecontrol fluid conducted to said valve means to effect operation of saidvalve means to the open condition and a porting of relatively lowpressure control fluid to said motor means in response to the occurrenceof an impending overloading of the prime mover.
 22. An apparatus as setforth in claim 21 further including manually actuatable means movablefrom an initial position to any one of a plurality of actuated positionsspaced different distances from said initial position, means foroperating said valve means from the closed condition to the opencondition in response to movement of said manually actuatable means fromthe initial position to any one of said actuated positions to portcontrol fluid to said motor means, and means for operating said valvemeans from the open condition to the closed condition in response tooperation of said motor means to an extent which is a function of theextent of movement of said manually actuatable means from the initialposition in the absence of an impending overloading of the prime mover.23. An apparatus as set forth in claim 22 wherein said means foroperating said vaLve means from the open condition to the closedcondition in response to operation of said motor means includes pressureresponsive means for operating said valve means to the closed conditionin response to an increase in the pressure of the control fluidconducted to said motor means to a pressure corresponding to the extentof movement of said manually actuatable means from the initial position,said anti-stall means being effective to prevent an increase in thepressure of the control fluid conducted to said motor means to apressure corresponding to the extent of movement of said manuallyactuatable means from the initial position in response to an impendingoverloading of the prime mover.
 24. An apparatus as set forth in claim21 wherein said anti-stall means includes means for maintaining thepressure of the control fluid conducted to said valve means at apressure which is greater than the pressure of the control fluidsupplied to said motor means and for varying the pressure of the controlfluid supplied to said valve means as a function of variations in thepressure of the control fluid supplied to said motor means.
 25. Anapparatus for controlling the operation of a hydrostatic transmissiondriven by a prime mover and having a variable displacement hydrostaticunit, said apparatus comprising fluid motor means for varying thedisplacement of the hydrostatic unit in response to variations incontrol fluid pressure to which said motor means is exposed, pressurecontroller means operable to anyone of a plurality of settings to varythe control fluid pressure to which said motor means is exposed withvariations in the setting of said pressure controller means, andanti-stall means connected in continuous fluid communication with saidmotor means through said pressure controller means for decreasing thepressure to which said motor means is exposed in response to theoccurrence of an impending overloading of the prime mvoer.
 26. Anapparatus for controlling the operation of a hydrostatic transmissiondriven by a prime mover and having a variable displacement hydrostaticunit, said apparatus comprising fluid motor means for varying thedisplacement of the hydrostatic unit in response to variations incontrol fluid pressure to which said motor means is exposed, pressurecontroller means operable to anyone of a plurality of settings to varythe control fluid pressure to which said motor means is exposed withvariations in the setting of said pressure controller means, andanti-stall means connected in fluid communication with said motor meansthrough said pressure controller means for decreasing the pressure towhich said motor means is exposed in response to the occurrence of animpending overloading of the prime mover, said pressure controller meansincluding valve means operable between a first condition varying thecontrol fluid pressure to which said motor means is exposed and a secondcondition in which the control fluid pressure to which said motor meansis exposed is maintained substantially constant and passage means forconnecting said anti-stall means in fluid communication with said motormeans when said valve means is in either the first condition or thesecond condition.
 27. An apparatus for controlling the operation of ahydrostatic transmission driven by a prime mover and having a variabledisplacement hydrostatic unit, said apparatus comprising fluid motormeans for varying the displacement of the hydrostatic unit in responseto variations in control fluid pressure to which said motor means isexposed, pressure controller means operable to anyone of a plurality ofsettings to vary the control fluid pressure to which said motor means isexposed with variations in the setting of said pressure controllermeans, anti-stall means connected in fluid communication with said motormeans through said pressure controller means for decreasing the pressureto which said motor means is exposed in response to the occurrence of animpending overloading of the prime mover, means for providing a firstsignal which varies with variations in the operating speed of the primemover, means for providing a second signal which varies with variationsin the setting of a throttle for the prime mover, and means forproviding a third signal which varies as a function of variations in thedisplacement of the hydrostatic unit, said anti-stall means includingmeans responsive to said first, second and third signals for reducingthe control fluid pressure to which said motor means is exposed upon theoccurrence of an impending overloading of the prime mover.
 28. Anapparatus for controlling the operation of a hydrostatic transmissiondriven by a prime mover and having a variable displacement hydrostaticunit, said apparatus comprising fluid motor means for varying thedisplacement of the hydrostatic unit in response to variations incontrol fluid pressure to which said motor means is exposed, pressurecontroller means operable to anyone of a plurality of settings to varythe control fluid pressure to which said motor means is exposed withvariations in the setting of said pressure controller means, andanti-stall means connected in fluid communication with said motor meansthrough said pressure controller means for decreasing the pressure towhich said motor means is exposed in response to the occurrence of animpending overloading of the prime mover, said anti-stall meansincluding means for maintaining the pressure of control fluid conductedto said pressure controller slightly above the control fluid pressure towhich said motor means is exposed for all displacements of saidhydrostatic unit in the absence of an impending overloading of the primemover.
 29. An apparatus for controlling the operation of a hydrostatictransmission driven by a prime mover and having a variable displacementhydrostatic unit, said apparatus comprising fluid motor means for varyinvarying displacement of the hydrostatic unit in response to variationsin control fluid pressure to which said motor means is exposed, pressurecontroller means operable to any one of a plurality of settings to varythe control fluid pressure to which said motor means is exposed withvariations in the setting of said pressure controller means, andanti-stall means connected in fluid communication with said motor meansthrough said pressure controller means for decreasing the pressure towhich said motor means is exposed in response to the occurrence of animpending overloading of the prime mover, said pressure controller meansincluding valve means for varying the control fluid pressure to whichsaid motor means is exposed, said valve means being operable between afirst condition varying the control fluid pressure to which said motormeans is exposed and a second condition in which the control fluidpressure to which said motor means is exposed is maintainedsubstantially constant, said apparatus further including fluid pressurefeedback means for urging operation of said valve means from the firstcondition to the second condition under the influence of a fluidpressure force which varies as a function of variations in the controlfluid pressure to which said motor means is exposed, and means forurging said valve means against operation to the second condition whicha second force with corresponds to a setting of said pressure controllermeans.